This giant ostracod lives at depths of up to roughly 6,500 feet (2,000 meters) in the North Atlantic Ocean.

Photograph by David Shale, Minden Pictures

Vampire squid also have a high eye-to-body ratio. The deep-dwelling cephalopods grow up to a foot (0.3 meter) long, with eyes about an inch (about 2.6 centimeters) wide. These big eyes are helpful at ocean depths of up to 8,200 feet (2,500 meters). (Related: "Vampire Squid’s Surprising Diet Revealed.")

Arachnids, Insects, and Tarsiers

The vampire squid can turn itself "inside out" to avoid predators.

Michael F. Land, co-author of the book Animal Eyes, agrees that since so many animals have large eyes, finding the biggest relative to size is a tall order.

But some nocturnal spiders, such as ogre-faced spiders, "come pretty high on the list," Land, a neurobiologist at the University of Sussex, says via email.

Of their eight eyes, the two largest, front-facing ones are enormous, helping the arachnids in their nighttime quest for prey.